In recent years, an apparatus that complies with a scheme such as MPEG (Moving Picture Experts Group) where image information is handled in digital form and at that time for the purpose of efficient transmission and accumulation of information, compression is performed by an orthogonal transform, such as a discrete cosine transform, and motion compensation, utilizing the redundancy unique to the image information has started to proliferate for both information distribution on broadcast stations, etc., and information reception in ordinary households.
In particular, MPEG2 (ISO (International Organization for Standardization)/IEC (International Electrotechnical Commission) 13818-2) is a standard defined as a general image coding scheme and covering both interlaced images and progressive images and standard definition images and high definition images, and is currently widely used in a wide range of applications for professional use and consumer use. By using the MPEG2 compression scheme, for example, by allocating a rate (bit rate) of 4 to 8 Mbps for a standard definition interlaced image with 720×480 pixels, and a rate (bit rate) of 18 to 22 Mbps for a high definition interlaced image with 1920×1088 pixels, a high compression rate and excellent image quality can be achieved.
MPEG2 is mainly targeted for high image quality coding compatible for broadcasting, but does not support a coding scheme with a lower rate (bit rate), i.e., a higher compression rate, than that of MPEG1. Due to the proliferation of portable terminals, the needs for such a coding scheme are expected to increase in the future, and correspondingly an MPEG4 coding scheme is standardized. For an image coding scheme, the standard thereof was approved as the International standard ISO/IEC 14496-2 in December 1998.
Furthermore, in recent years, for the initial purpose of image coding for videoconference, standardization of the standard called H.26L (ITU-T (International Telecommunication Union Telecommunication Standardization Sector) Q6/16 VCEG (Video Coding Expert Group)) has been pursued. It is known that although H.26L requires a larger amount of computation for its coding and decoding compared to conventional coding schemes such as MPEG2 and MPEG4, higher coding efficiency is achieved. In addition, at present, as part of MPEG4 activities, standardization that achieves higher coding efficiency by adopting, on the basis of this H.26L, functions not supported by H.26L is performed as Joint Model of Enhanced-Compression Video Coding.
For the standardization schedule, the standard became an international standard in March 2003 in the name of H.264 and MPEG-4 Part10 (Advanced Video Coding, hereinafter, referred to as AVC).
However, a macroblock size of 16 pixels×16 pixels may not be optimum for a large picture frame, like UHD (Ultra High Definition; 4000 pixels×2000 pixels) which may serve as a target for a next-generation coding scheme.
Hence, currently, for the purpose of further improving coding efficiency over AVC, standardization of a coding scheme called HEVC (High Efficiency Video Coding) is pursued by JCTVC (Joint Collaboration Team-Video Coding) which is a joint standardization organization of ITU-T and ISO/IEC (see, for example, Non-Patent Document 1).
In the HEVC coding scheme, a Coding Unit (CU) is defined as a processing unit similar to a macroblock in AVC. The size of the CU is not fixed at 16×16 pixels, like a macroblock in AVC, and is specified in image compression information in each sequence.
Meanwhile, for an Adaptive Loop Filter (ALF) used in such a coding scheme, the Wiener filter is applied for a reconstructed image to remove noise contained in the reconstructed image, enabling to improve coding efficiency and image quality.
As one of the techniques using ALF, there is proposed a method called class classification ALF where filter characteristics are switched using information that can be classified into a class by a decoder, to allow the filter characteristics to suit local conditions. For information used for class classification, in information used for class classification which is considered in the next-generation video coding standard HEVC, there is the magnitude of SML (Sum-Modified Laplacian) which is an operator for obtaining the complexity of texture. SML is calculated using the differences between a focused pixel and neighboring pixels.
In addition, HEVC adopts a method called an adaptive offset filter which is proposed in Non-Patent Document 2. In HEVC, the adaptive offset filter is provided between a deblocking filter and an adaptive loop filter.
For the kinds of adaptive offset, there are of two kinds called band offset and six kinds called edge offset, and furthermore, it is also possible not to adapt offset. Then, an image is partitioned into a quad-tree, and a selection can be made for each region as to which one of the above-described kinds of adaptive offset is to be used for coding. By using this method, coding efficiency can be improved.
In such edge offset of the adaptive offset filter, neighboring pixel values are referred to for calculation of an offset value for a focused pixel.
Meanwhile, in the above-described image coding schemes such as AVC and HEVC, for example, a method is prepared in which in order to parallelize processes, a picture is divided into a plurality of slices and a process is performed on a slice-by-slice basis.